Legal child can buy a DTV as dependant, you will apply for the child referencing your DTV number/approval as parent and supply a sponsorship letter and bank statement
Destination Thailand Visa (DTV) join and read. Thai massage training hasn't been a scheme seeing any approvals, but that is most likely due to proper and acceptable certification. Contacting schools with ministry of education certification as well as tourism certification would be advised, of course if they have a history of DTV application approval as well.
Avoid delays and disappointment, say anywhere and include Thailand specifically and also mention that you will not interact with Thai businesses or individuals.
What do you mean at boarder? The house owner/land owner does the TM30 to report a foreigner staying at their house. It is to be reported within 24 hrs of the foreigner using the accommodation.